Orthogonal frequency division multiplexing (OFDM) is considered a prime candidate for the fourth generation (4G) and the super third generation (S3G) radio access systems. This choice is partly based on the effective utilization of the available bandwidth compared to other multiplexing techniques. In OFDM, the available bandwidth is subdivided into many narrow bandwidth sub-carriers. These sub-carriers are orthogonal to each other, thus allowing them to be packed together much closer than standard frequency division multiplexing (FDM) and providing a higher spectral efficiency. Each user may be allocated several sub-carriers in which to transmit their data, allowing multiple data symbols to be sent in parallel. In other words, a transmitted OFDM signal multiplexes several low-rate data streams, where each data stream is associated with a given sub-carrier. A main advantage with this concept is that each of the data streams experiences an almost flat fading channel.
When an OFDM signal is transmitted over a dispersive channel, the channel dispersion destroys the orthogonality between sub-carriers and causes intercarrier interference (ICI). In addition, a system may transmit multiple OFDM symbols in series so that a dispersive channel causes inter-symbol interference (ISI) between successive OFDM symbols. The orthogonality of the sub-carriers can be preserved and ICI and ISI can be prevented by employing so-called cyclic extensions inserted between consecutive OFDM symbols [1, 2].
As long as the duration of the channel impulse response is less than the length of the cyclic extension, channel equalization can be performed with just a single complex multiplication per sub-carrier and ISI and ICI are prevented. If the channel impulse-response exceeds the cyclic extension length, both ISI and ICI will occur.
Today OFDM is used in digital terrestrial broadcast systems such as Digital Video Broadcasting (DVB) and Digital Audio Broadcasting (DAB). One reason for selecting OFDM for digital broadcast is the possibility to operate the system in a Single Frequency Network (SFN) mode. In SFN mode, the same radio signal is transmitted from all radio transmitter nodes in the network. As long as the transmitted radio signals in the network are received by a receiver terminal within a time window less than the length of the cyclic extension, the resulting signal can be detected without ISI or ICI using a very simple channel equalizer. The signals from the different transmitters are combined at the receiver terminal in the same way as different multi-path signals are combined.
The cyclic extension is typically discarded in the receiver terminal and, hence, it is associated with an overhead cost that should be kept as small as possible. Typically the length of the channel impulse response and, thus, of the cyclic extension depends on the maximum expected distance between the transmitter node(s) and the receiver terminal. The longer the distance the longer cyclic extension is in general needed.
When employing the SFN technique for information that shall be broadcast to multiple receiver terminals, the required cyclic extension must typically be longer than what is needed for normal cellular operation where node-specific information is transmitted between a single transmitter node and a receiver terminal in a given cell.
In order to enable efficient support of broadcast information, OFDM symbols with long associated cyclic extensions for broadcast in SFN operation mode are time multiplexed with symbols having shorter cyclic extensions for normal node-specific operation (unicasting). The so-obtained semi-fixed time pattern or allocation structure of multiplexed symbols of long and short cyclic extensions, respectively, are then employed by all transmitter nodes throughout the network.